First, a description will be made of a dispersion shift single-mode optical fiber.
The usually used dispersion-shifted single-mode optical fiber (hereinafter simply referred to as a dispersion-shifted optical fiber or a DSF) having an outer diameter of 125 .mu.m has at its center a core part having a higher refractive-index than that of its circumference. When viewing the construction of this DSF from the standpoint of the distribution of the intensity of the light, the intensity of the light is the largest at the center of the DSF, and the intensity of the light becomes smaller as the light approaches the outer circumference away from the center. The change of that intensity is of a sharp bell-shaped curve. Usually, in such a case, the center portion of the DSF is called as the "mode field", and the diameter thereof is called the "mode field diameter". Note that, although the mode field diameter slightly differs according to some conditions, when the wavelength of the light which is propagated is 1.55 .mu.m, the general construction of the DSF has a mode field diameter of about 8 .mu.m.
A preform for a DSF having a construction as mentioned before is produced (manufactured) as follows. (a) First, a porous glass body of a core rod for a DSF including a core portion in which at least a metal dopant exists is synthesized by a VAD method etc (at present, frequently, part of the cladding portion is synthesized together with the core portion, and hereinafter, one including also a part of this cladding portion is referred to as a core rod for the DSF). (b) Subsequently, the porous glass body is dehydrated and sintered to form the core rod for the DSF. (c) Further, using the core rod for the DSF as a target rod, a porous glass layer having a desired thickness corresponding to the cladding layer is formed on the outer circumference thereof by an outer vapor deposition (OVD method) etc., then (d) that porous glass layer is dehydrated and formed into a glass, thereby to obtain a preform for the DSF. (e) Thereafter, that preform for the DSF is heated and drawn, thereby to obtain a DSF having a desired thickness.
In recent years, due to the advances made in various technologies, it has become possible to obtain a large size preform for a DSF. In this case, among the aforementioned manufacturing steps, the manufacturing step of forming the porous glass layer by the OVD method or the like and the manufacturing step of dehydration and glass-forming of the porous glass are often repeated to fabricate the large sized preform for a DSF.
The above conventional manufacturing method of a large size DSF is characterized in that the transmission characteristic is easily controlled. No superior manufacturing method to take the place of this has yet been found.
However, in the above manufacturing method, the manufacturing steps for making a single preform comprise at least two different manufacturing steps of the VAD method and OVD method. For this reason, a boundary surface exists between the glasses manufactured by the different steps. It is known that defects are apt to occur at the boundary surface. Also, glass has a different viscosity according to the content of impurities. Usually, the purity of the glass existing on the outer circumference of the core rod portion formed by the OVD method or the like tends to be low in comparison with the purity of the core rod portion. Also, in the core rod, the concentration of the metal dopant in the diameter direction is not constant since the distribution of refractive-index is controlled. For this reason, the viscosity of the glass of the dispersion-shifted optical fiber is not uniform.
The defects of the glass mentioned before exert the following effects upon the DSF. For example, when the DSF manufactured by the conventional method is left to stand in a hydrogen atmosphere for a long time, the hydrogen disperses in the DSF. Next, the dispersed hydrogen enters into the defects of the glass and forms Si-H bonds. The Si-H bonds absorb the light having a wavelength of 1.52 .mu.m among the light propagated through the DSF and causes a loss, i.e., causes a so-called increase of hydrogen loss, so the transmission loss becomes large.
Moreover, hydrogen exists also in the atmosphere, and therefore if there is such a tendency in the DSF, even in a case where the DSF is left to stand in the atmosphere, it is clear that the DSF will be influenced in some way. In view of this situation, a DSF little effected by defects at the boundary surface of the glasses formed by the different manufacturing steps has been required.
Further, in the optical fiber preform of a DSF, the purity of the glass formed by the OVD method is lower than that of the core rod portion. Also, the concentration of the metal dopant is not constant due to the distribution of the refractive-index in the diameter direction in the core rod. Particularly, where a cladding layer not containing a metal dopant is formed at the same time as that for the core portion in the outer layer portion of the core rod, from the center of the optical fiber, three concentric circular parts of a layer having a high purity and containing a metal dopant, a layer having a high purity and not containing the metal dopant, and a layer having a low purity and substantially not containing the metal dopant are formed. The glass containing a metal dopant and impurities has a lower viscosity in comparison with the glasses not containing them and therefore exhibits a three-concentric circle construction including a part having a low viscosity, a part having a high viscosity, and a part of a low viscosity corresponding to the construction of the DSF.
When drawing the preform of DSF having such a distribution of viscosity, a tensile strain is added to the part where the viscosity is high. For this reason, distortion remains in the DSF after drawing and the strength of the DSF was sometimes greatly lowered.
Next, a description will be made of the conventional single-mode optical fiber.
The usually used single-mode optical fiber having an outer diameter of 125 .mu.m has a core having a diameter of about 10 .mu.m at the center and a cladding having a diameter of 125 .mu.m formed on the outer circumference thereof. The refractive-index of the core at the center of the single-mode optical fiber is higher than the refractive-index of the cladding on the circumference thereof. Viewing the construction of this single-mode optical fiber from the standpoint of the distribution of the intensity of light, the intensity of the light is the largest at the center (core) of the optical fiber, and the intensity of the light becomes smaller as the light approaches the outer circumference away from the center. Namely, a sharp bell shaped curve distribution is exhibited. Usually, in such a case, the center portion of the single-mode optical fiber is called the "mode filed", and the diameter thereof is called the "mode field diameter". Note that, although the mode field diameter slightly differs according to some conditions, when the wavelength of the light to be propagated is 1.55 .mu.m, the general construction of the single-mode optical fiber has a mode field diameter of about 9 to 11 .mu.m.
A preform for the single-mode optical fiber having a construction as mentioned before is manufactured as follows: (1) First, a porous glass body of a core rod for the optical fiber including at least the core portion is synthesized by the VAD method etc (at present, frequently, part of the cladding portion is synthesized together with the core portion, and hereinafter, one including also a part of this cladding portion is referred to as a core rod for the optical fiber). (2) Subsequently, the porous glass body is dehydrated and sintered to form the core rod for the optical fiber. (3) Further, the core rod for the optical fiber is used as a target rod, (4) a porous glass layer having a desired thickness corresponding to the cladding layer is formed on the outer circumference thereof by the OVD method or the like, and then (5) that porous glass layer is dehydrated and formed into a glass, thereby to obtain the preform for the optical fiber.
In recent years, due to the advances made in various technologies, it has become possible to obtain a large-size preform for an optical fiber. In this case, among the aforementioned manufacturing steps, the manufacturing step of forming the porous glass layer by the OVD method or the like and the dehydration and glass-forming step of the porous glass are repeated to manufacture the large-scale preform for the optical fiber.
The aforementioned conventional manufacturing method of a large-size optical fiber is characterized in that the distribution of the refractive-index at the center part is easily controlled. A manufacturing method to take the place of this has not yet been found at present.
However, in the above manufacturing method, the manufacturing steps for forming a single preform comprise at least two different manufacturing steps of the VAD method and OVD method. For this reason, a boundary surface exists between the glasses manufactured by the different steps. It is known that defects are apt to occur at the boundary surface.
The above defects of the glass exert the following effects upon the optical fiber. For example, when a single-mode optical fiber obtained by drawing a preform for an optical fiber manufactured by the conventional method is left to stand in a hydrogen atmosphere for a long time, the hydrogen disperses in the optical fiber. Next, the dispersed hydrogen enters into the defects of the glass and forms Si-H bonds. The Si-H bonds cause an increase of the so-called hydrogen loss, i.e., absorb the light having a wavelength of 1.52 .mu.m among the light propagated through the optical fiber, and thus the transmission loss becomes large.
Moreover, hydrogen exists also in the atmosphere, and therefore if there is such a tendency in the optical fiber, it is clear that, even in a case where the single-mode optical fiber is left to stand in the atmosphere, the single-mode optical fiber will be influenced in some way.